In the prior art, the device management system is configured to manage a plurality of devices. (“manage” means “at least one of the “monitor” and “control”.) A plurality of the devices belong to a plurality of the management groups, respectively. The management groups are categorized on the basis of a sort of the devices and a place of the devices. Consequently, the device management system constructs inferior communication networks. A plurality of the devices belong to a plurality of the management groups, respectively. The inferior communication networks are managed by the ancestor communication network. When these communication networks construct the hierarchized communication network, the technology of managing sub units by a main unit is employed. In this technology, the sub units are arranged in the inferior communication network so as to manage devices, and the main units are arranged in the superior communication network so as to include the sub units as terminals. (The sub unit includes “a sub unit with the device”, in the broad sense.) The main unit is configured to communicate with the sub unit with using a communication protocol. The communication protocol is exemplified by BACnet in the Japanese patent application publication No. 2007-96539. (BACnet is “A Data Communication Protocol for Building Automation and Control Networks”.)
However, in the device management system of the prior art, if the main unit is failed, the main unit having a failure is not capable of managing the sub units which are managed by the main unit. As a result, the prior device management system has a problem of that it is impossible to manage the devices under the sub unit which is managed by the main unit having the failure.